1. Field of the Invention
The present invention generally relates to an image fixing apparatus and an image forming apparatus equipped with the fixing apparatus, and more particularly to an image fixing apparatus capable of fixing a toner image onto a recording medium by effectively controlling an image fixing temperature in an electrophotographic process.
2. Discussion of the Background
A background image forming apparatus such as a printer, a copying machine, and a facsimile using an electrophotographic method is generally provided with an image fixing apparatus that melts and fixes a toner image onto a recording medium with heat. This heating type image fixing apparatus commonly includes a heater, a fixing member, and a rotating member. The heater is energized with power to produce heat. The fixing member is heated up to a target temperature by the heater. The rotating member is arranged in contact with the fixing member to form a nip region therebetween. Since the heater keeps the nip region at the target temperature, a recording medium carrying a toner layer thereon undergoes an image fixing process as it passes through the nip region.
Stably maintaining the target temperature is a key element of good quality image formation. If the temperature is higher or lower than the target, a poor image, referred to as an offset image, may be formed or poor fixing may occur. Furthermore, reducing a warm-up period of the image fixing apparatus is also needed for shortening a waiting time.
One exemplary attempt to shorten the warm-up time is to reduce a heat capacity of the fixing member. This attempt produces a high heat nip between the fixing member, in a form of a thin film having a relatively low heat capacity, and the rotating member, while achieving a reduction of warm-up time. This attempt also achieves a successful use of a belt-shaped fixing member. In this case, the fixing belt is extended between two or more rollers including a first roller having a relatively low thermal conductivity and a second roller serving as a heat source. The first roller closely faces a third roller via the fixing belt to form a nip with the fixing belt.
In addition to these low heat capacity examples, the fixing member having a low heat capacity may be provided with an alternative heat source, that is, an induction heater for directly heating the fixing member.
The above techniques, however, require a high accuracy in controlling temperature of the fixing member due to its low heat capacity.
Controlling electric power to the heater controls temperature of the fixing member. For controlling electric power, temperature sensors are provided in the fixing apparatus such as a thermo pile and a thermo sensitive register to detect a temperature of the fixing member. When the detected temperature is lower than the target temperature, the heater is turned on. When the detected temperature is higher than the target temperature, the heater is turned off. This is called an ON-OFF controlling method.
Although the ON-OFF controlling is used in this method, carrying out precise temperature control may be difficult due to a temperature ripple.
A temperature ripple induces to use a compensation in which a calculator calculates an amount of operations and outputs the amount to a controlled object based on the desired temperature value and the amount of feedbacks. For example, the amount of proportional integral (PI) or proportional integral differential (PID) operations is calculated as the compensation.
Even if the control method with the compensation is used, an exothermic delay in the heater, a heat transmission delay from the heater to a fixing member surface, a detecting delay from a slow response of a temperature sensor, and a delay from a heater driver to the temperature sensor may occur. They may cause a temperature ripple. Furthermore, the temperature ripple may easily occur in a belt fixing apparatus, because the heat delay in the belt is greater.
For reducing the temperature ripple, two or more output patterns of a specific pulse corresponding to a detected temperature may be stored and one of the control patterns may be selected according to the detected temperature to carry out the temperature control. As another method, calculating the change rate of difference between the last control timing temperature and the present detected temperature is used for predicting the next control timing temperature to control the temperature. The number of the specific pulses or the width of the specific pulse is used for the control.